1. Field of the Invention
This invention relates to computer systems, and more particularly, to methods and apparatus for providing a precise regulated voltage from a charge pump while expending an extremely low amount of power.
2. History of the Prior Art
There has been a recent trend toward lowering the power requirements of portable computers. In order to reduce power consumption, much of the integrated circuitry used in personal computers is being redesigned to run at low voltage levels. The circuitry and components used in portable computers are being designed to operate at voltage levels such as 5 volts and 3.3 volts. This helps a great deal to reduce the power needs of such computers.
However, some of the features of portable computers require voltages higher than these supply voltages. For example, flash electrically-erasable programmable read only memory (flash EEPROM memory) has begun to be used as a new form of long term random access storage. An example of a flash EEPROM memory array which may be used in place of a hard disk drive is described in U.S. patent application Ser. No. 07/969,131, entitled A Method and Circuitry For A Solid State Memory Disk, S. Wells, filed Oct. 31, 1992, and assigned to the assignee of the present invention. Such an array provides a smaller lighter functional equivalent of a hard disk drive which operates more rapidly and is not as sensitive to physical damage. Such memory arrays are especially useful in portable computers where space is at a premium and weight is important. These flash EEPROM memory arrays, however, require much higher voltages and substantially more power for programming and erasing data than can be provided directly by the batteries of low powered portable computers.
Charge pumps may be used in portable computers to provide a high voltage from a lower voltage source. However, one problem with using charge pumps is that the voltage level provided at an output terminal tends to vary substantially from a desired value. The output of a charge pump is furnished in a series of charge pulses which are stored to provide a source voltage. The charge pump generates this form of output because the output stage functions in the manner of a diode which only transfers charge to the output in a properly biased condition. This creates a voltage ripple on the output of the pump when the current requirements are high relative to the capacitance of the load. Moreover, the output voltage will vary for different source voltages, temperatures, manufacturing processes, and load currents. On the other hand, it is necessary to provide very accurate voltages for programming and erasing the cells of a flash EEPROM memory array. This problem makes it desirable to attempt to regulate the output voltage provided by charge pumps to obtain a relatively constant value over ranges of source voltages, temperatures, and manufacturing processes.
Another way in which the power requirements of the flash EEPROM arrays have been reduced is through the use of negative gate erase techniques. These techniques reduce the amount of current required during the erase of the flash EEPROM memory array and thereby reduce the power used. Consequently, it is especially desirable to regulate the output voltage provided by charge pumps utilized to generate voltages used for negative gate erase operations.
One voltage necessary for use with flash EEPROM memory arrays is a negative voltage which is used at the gate terminal of a P channel isolation transistor which is part of the word decoder in such memory arrays. Such a transistor must be operated with very little delay once an operation is commenced. For this reason, the negative voltage used must be immediately available. Providing art immediately available voltage from a charge pump normally requires that the charge pump be on at all times. This, in itself, usually requires the expenditure of substantial power. It is, therefore, desirable to provide an ultra-low power negative charge pump circuit which may remain operative while expending very little power.